A frame is a data sequence channel having a predetermined period of time in terms of physical characteristics and includes a downlink (DL) subframe and an uplink (UL) subframe. A preamble is specific sequence data located at a first symbol of each frame and is used for synchronization of a terminal or Mobile Station (MS) to a Base Station (BS) or used for channel estimation. A Frame Control Header (FCH) provides DL-MAP-related channel allocation information and channel code information. The DL-MAP/UL-MAP is a Media Access Control (MAC) message used to notify an MS of channel resource allocation in uplink/downlink. A Downlink Channel Descriptor (DCD)/Uplink Channel Descriptor (UCD) message is a MAC message used for notification of physical characteristics in a downlink/uplink channel. A data burst is a unit of data transmitted to or received by an MS. Notification of the size and position of each burst is provided through a DL/UL-MAP message.
FIG. 1 illustrates an example frame structure used in a broadband wireless access system of IEEE 802.16e.
One frame in the broadband wireless access system, for example a 5 ms frame in WiMAX, is divided into one DL subframe and one UL subframe. The DL frame includes a preamble, an FCH, a DL-MAP, a UL-MAP, and DL bursts and the UL frame includes UL control channels such as an HARQ ACK channel, a fast-feedback channel, and a ranging channel and UL bursts.
A new broadband wireless access system such as IEEE 802.16m/IMT-Advanced is under development to improve the broadband wireless access system of IEEE 802.16e. IEEE 802.16m introduces superframe and subframe structures, unlike the conventional frame structure.
FIG. 2 illustrates example superframe, frame, and subframe structures in IEEE 802.16m.
In the example of FIG. 2, one superframe is 20 ms long and includes 4 frames, each 5 ms long. A superframe MAP, which is a message including system information and broadcast messages, is provided at the beginning of each superframe. The superframe MAP may have a structure including several symbols or a structure including several subframes. One 5 ms-long frame includes 8 subframes and one subframe includes 6 OFDMA symbols.
FIG. 3 illustrates a subframe structure in which the ratio of the number of DL subframes to the number of UL subframes is set to 5:3 in a TDD system.
One frame structure including 8 subframes may include 5 DL subframes and 3 UL subframes.
FIG. 4 illustrates DL and UL-related control channels in the subframe structure of FIG. 3.
In the example of FIG. 4, a preamble is located at the beginning of each frame and a DL sub-MAP is located at each DL subframe. The DL sub-MAP is located subsequent to the preamble in the first DL subframe SF#0 and is located at the beginning of each of the remaining subframes SF#1-4. The DL sub-MAP mainly includes resource allocation information in the subframe including the DL sub-MAP and additionally includes subframe configuration information or DL system information. A UL sub-MAP may be located subsequent to the DL sub-MAP.
In FIG. 4, the UL sub-MAP is located subsequent to the DL sub-MAP in each of the second, third, and fourth subframes SF#1, SF#2, and SF#3.
The UL sub-MAPs of the second DL subframe SF#1, the third DL subframe SF#2, and the fourth DL subframe SF#3 include resource allocation information corresponding to the sixth DL subframe SF#5, the seventh DL subframe SF#6, and the eighth DL subframe SF#7, respectively. Each of the UL subframes SF#5, SF#6, and SF#7 may include UL control channels associated with the UL subframe such as an HARQ ACK/NACK channel, a fast feedback channel, and a ranging channel.
Two or more DL subframes or two or more UL subframes may be grouped according to scheduling of the BS.
FIG. 5 schematically illustrates a handover procedure when an MS has issued a handover request to a BS.
A serving BS broadcasts information including network structure information to all MSs in a cell through a neighbor advertisement (MOB_NBR-ADV) MAC message to notify all the MSs of information of neighbor BSs.
A Mobile Station (MS) scans neighbor BSs for handover and performs handover based on channel quality information of channels established with neighbor BSs obtained through scanning.
Handover can be initiated by both the BS and the MS. In the case where the MS requests handover, the MS transmits a handover request (MOB-MSHO-REQ) MAC message to the BS (510). The handover request (MOB-MSHO-REQ) MAC message includes channel status information of signals received from neighbor BSs.
The serving BS receives an ACK signal required to perform handover to a target BS from each neighbor BS before the serving BS transmits a handover response (MOB-BSHO-RSP) MAC message to allow the serving BS or the MS to perform handover.
The serving BS then transmits a handover response (MOB-BSHO-RSP) MAC message to the MS (540).
The MS transmits information of a target BS to which handover is to be performed to the serving BS through a handover indicator (MOB_HO-IND) (550).
The serving BS transmits information indicating handover of the MS to the target BS (560) and terminates an ARQ connection allocated to the MS and all connections associated with data transmission.
In order to perform fast handover of the MS, the target BS can transmit a fast ranging Information Element (IE), which uses a handover identifier (HO ID) or a MAC address of the MS, to the MS (570), thereby omitting a CDMA code ranging procedure. In general, when the MS needs to perform handover, the MS transmits a handover CDMA ranging code to the target BS. When successful ranging is possible, the target BS broadcasts a ranging response (RNG-RSP) MAC message and transmits a CDMA allocation IE to cause the MS to transmit a ranging request (RNG-REQ) MAC message.
Upon receiving the fast ranging IE or the CDMA allocation IE, the MS transmits a ranging request (RNG-REQ) MAC message to the target BS (590). The ranging request (RNG-REQ) MAC message includes a handover identifier (HO ID) or MAC address of the MS, CMAC information for authentication of the MS, etc.
Upon receiving the ranging request (RNG-REQ) MAC message, the BS generates a Connection ID (CID) of the MS and transmits a ranging request (RNG-RSP) MAC message to the MS. Here, the BS can notify the MS of whether or not to omit renegotiation and re-authentication/re-registration processes of the MS through a handover process optimization field in the ranging request (RNG-RSP) MAC message. Upon receiving the ranging request (RNG-RSP) MAC message, the MS can omit some processes through the handover process optimization.
In FIG. 6, the TTIs of first, second, and third subframes are each one subframe (i.e., TTI=1 subframe) and fourth and fifth subframes are grouped to construct one TTI (i.e., TTI=2 subframes). Since information indicating whether or not each subframe has been grouped or TTI information is transmitted in a superframe MAP, an MS decodes, when attempting to perform handover, received signals according to a default TTI (for example, TTI=1 subframe) during up to 15 ms until a superframe MAP is received from a target BS. This increases interruption time for handover.
Particularly, in the case where an MS, which is attempting to perform handover, receives a signal from the target BS, beginning with a second frame, as shown in FIG. 7, the MS will attempt to decode the signal according to a 1-subframe TTI format, thus failing to decode a data burst allocated to the frame.